Antiskid brake control systems have been in widespread use for many years. In the simplest sense, an antiskid brake control system compares the speed of a vehicle derived from a wheel speed sensor (and wheel radius) to the vehicle speed derived from a secondary or reference source. If the wheel is determined to be skidding an excessive amount, then brake pressure applied to the wheel is released and the wheel is allowed to spin back up to the appropriate speed.
There are, of course, two major problems that immediately become apparent in any such antiskid system. The first relates to determining the appropriate amount of skidding. The second relates to determining from where to obtain the reference velocity. The appropriate amount of skidding is described by the much discussed but seldom measured mu-slip curve. Typically such curve is represented by the coefficient of friction .mu. between the wheel and the running surface on a vertical axis and the slip ratio on the horizontal axis. A slip ratio of zero is when the wheel is not skidding while a slip ratio equal to one represents a fully locked wheel.
The amplitude and peak location of the mu-slip curve unfortunately can vary substantially for different running surfaces or even the same running surface. A lower amplitude mu-slip curve may represent an ice or water patch. Ideally, the antiskid brake control system should allow the wheel to slip at the peak of the mu-slip curve which provides the maximum stopping power. Antiskid brake control systems are commonly accepted to be ninety percent efficient which means that, on average, the control system should be within ten percent of the mu-slip peak regardless of the value or location of the peak. However, since the mu-slip curve depends on so many variables (e.g., and without being limited thereby, tire tread groove pattern, tire tread compound, temperature, tire pressure, running surface material and finish, etc.), the mu-slip curve begins to resemble a random variable. This makes it difficult for conventional antiskid brake control systems to track adequately the peak of the mu-slip curve.
Furthermore, it is not always easy to obtain a reference velocity of the vehicle. The braked wheel cannot be used freely because the wheel might be skidding. In the case of an aircraft brake control system, the nose wheel speed could be used for providing a reference velocity. Unfortunately, there could be instances where the nose wheel has not touched down yet or perhaps is worn or has low pressure so as to have a reduced radius. Alternatively, for example, the nose wheel may be locked due to a defective bearing. These instances will result in an erroneous reference velocity. A global positioning satellite (GPS) system or Doppler radar could provide a reference velocity but it may be inoperable or jammed. Alternatively, an inertial navigation system might provide a reference velocity. However, such systems can be prone to measurement error and/or relatively high cost.
Recently there have been efforts to utilize optimal state estimation techniques, such as Kalman filters, in antiskid brake control systems. For example, U.S. Pat. No. 4,679,866 to van Zanten et al. discusses a method for ascertaining a set-point braking moment using a Kalman filter. U.S. Pat. No. 4,715,662 to van Zanten et al. describes a method for determining an optimal slip value using a Kalman filter. While such systems attempt to take advantage of state estimation techniques in determining the optimal slip value, etc., there still remain difficulties as far as observability, speed of convergence, process and measurement noise, etc. Matters such as these can greatly affect the ability of such a system to actually operate in practice.
In view of the aforementioned shortcomings associated with conventional antiskid brake control systems, there is a strong need in the art for a system capable of accurately and reliably ascertaining the relevant parameters such as the appropriate amount of skidding and reference velocity. Moreover, there is a strong need in the art for a system which can ascertain such information more rapidly and accurately as compared to conventional systems.